A primary requirement to be met by a microphone for measurement and scientific purposes is that the acoustic performance of the microphone must be good, meaning that, in order to achieve good accuracy of the measurement, the linearity and stability of the microphone are good, and that the microphone disturbs or affects the sound field to be measured in a well controlled and predictable way. It is further necessary that the microphone have a low sensitivity to environmental variations such as temperature and static pressure. In order to obtain reproducible results and to extend the intervals between calibrations it is also imperative that the microphone exhibits good short-term and long-term stability. Furthermore it must be possible to carry out calibration in a simple manner to verify the primary characteristics of the microphone, which are its frequency response and sensitivity. Furthermore it must be possible to predict the performance of the microphone not only by means of direct measurements, but also by means of calculations based on theoretical considerations in order to give an independent confirmation of the measured signal.
Condenser microphones for scientific and measurement purposes are commonly made up of precision-machined mechanical elements. The main elements of a condenser microphone are a stationary electrode, also called a back plate electrode, and a movable electrode embodied as a diaphragm which, when at rest, is kept at a well-defined distance from the back plate electrode. The back plate electrode and the diaphragm are, and together they constitute the electrodes of a capacitor employing ordinary atmospheric air as the dielectric. The diaphragm, which in high quality transducers is made of metal, is usually mounted at an end of the microphone housing. The microphone housing, the insulator and the diaphragm form a closed compartment. The occurrence of a pressure difference between the outer atmosphere and the closed compartment causes the diaphragm to move, and this movement causes a change in capacitance, which can be measured electrically. The frequency response at higher frequencies is determined essentially by the resonance of the diaphragm and by its damping. The resonance frequency is determined by the mass of the diaphragm and by its mechanical tension. The damping depends on the mobility of the air in the space between the diaphragm and the back plate electrode, and therefore it can be varied and controlled by varying the geometry of the back plate electrode and by choosing the appropriate distance between the diaphragm and the back plate electrode. In most measurement microphones the distance between the diaphragm and the back plate electrode typically ranges from 10 μm to 30 μm. For an individual type the tolerance of the distance between the diaphragm and the back plate electrode must be controlled within ±5% in order to get a uniform damping of the diaphragm displacement in the region of interest. The damping is usually controlled by having a number of holes in the back plate electrode, which lead from the space between the diaphragm and the back plate electrode to the rear surface of the back plate electrode. The sensitivity of a condenser microphone is proportional to the distance between the electrodes and inversely proportional to the tension in the diaphragm. As the tension is dependent on the extension of the foil, the diaphragm has to be fixed to the microphone housing or ring-shaped member in a very well defined manner in order to have a good long-term stability.
GB 2 112 605 discloses a prior art condenser microphone, which is shown in FIG. 1. The prior art microphone has a cylindrical microphone housing with a transversal wall supporting an inner cylindrical wall coaxially with the microphone housing. A ring-shaped disc of an insulating material is press-fitted into the opening of the inner cylindrical wall. A coating layer of an electrically conductive material covers the central portion of the upper surface of the insulating disc and is spaced from the inner cylindrical wall. The conductive material also covers the surface in the opening in the ring-shaped disc, where a conductor is connected to the coating. The wire is connected to a terminal of the microphone, which is insulated from the housing. A conductive diaphragm is mounted over the end of the housing at a small distance from the coating on the ring-shaped disc.
The prior art microphone in FIG. 1 has some fundamental problems, which are that the entire microphone must be assembled before test and characterisation are possible, and that all parts in the microphone have a great influence on the sensitivity of the microphone to temperature, meaning that the materials and dimensions of all included parts must be selected with great care. Also the prior art microphone is costly to produce, while the invention provides a simple design, simple manufacture and lower price.
EP 371 620 discloses a typical microphone for lower cost applications. That construction has eliminated the need for a separate stationary electrode or back plate electrode by integrating the stationary electrode into the housing. While this is an elegant way of reducing the number of components in low cost microphones, it is unsuited for measurement microphones for many reasons. Among these are that in measurement microphone requirements with respect to tolerances require that the distance between the two electrodes is controlled within ±5%, which is not possible in this design; i.e. if for example the microphone is subjected to a mechanical shock resulting from being accidentally dropped onto a floor, the housing might deform, causing the distance between the diaphragm and stationary electrode to change. Also, scientific and measurement microphones must have very low sensitivity to variations in temperature, humidity and static pressure and this is difficult to achieve in this design. Also measurement microphones require that it must be possible to predict the microphone performance by means of calculations based on theoretical considerations in order to give an independent confirmation of the measured data, and this will be difficult in this design.